September 12, 2012

Carbon Dioxide Snowfall Seen In Martian Atmosphere

Although lately it has been snapping images of Curiosity and its landing remnants, NASA's Mars Reconnaissance Orbiter (MRO) data has given scientists the clearest evidence yet of carbon dioxide snowfalls on Mars.

Frozen carbon dioxide, or "dry ice," requires temperatures of about negative 193 degrees Fahrenheit, which is considered colder than needed for freezing water.

Carbon dioxide snow reminds scientists that although some parts of Mars may look similar to Earth, the planet is much different.

"These are the first definitive detections of carbon dioxide snow clouds," said lead author Paul Hayne of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "We firmly establish the clouds are composed of carbon dioxide -- flakes of Martian air -- and they are thick enough to result in snowfall accumulation at the surface."

The snow falls occurred from clouds around Mars' south pole in winter, and it was first observed by NASA's Phoenix Lander mission in 2008.

The scientists wrote in the Journal of Geophysical Research that they analyzed data gained by looking at clouds straight overhead, and sideways with the Mars Climate Sounder. This is one of six instruments onboard MRO.

This instrument records brightness in nine wavebands of visible and infrared light as a way to examine particles and gases in the Martian atmosphere.

The data helps to provide scientists with information about temperatures, particle sizes and their concentrations. The new analysis is based on data from observations in the south polar region during southern Mars winter in 2006 through 2007.

During this time, a tall carbon dioxide cloud about 300 miles in diameter was hovering over the pole, and smaller, lower-altitude carbon dioxide ice clouds were seen at latitudes from 70 to 80 degrees south.

"One line of evidence for snow is that the carbon dioxide ice particles in the clouds are large enough to fall to the ground during the lifespan of the clouds," said David Kass of NASA's Jet Propulsion Laboratory. "Another comes from observations when the instrument is pointed toward the horizon, instead of down at the surface."

He said the infrared spectra signature of the clouds view at this perspective is carbon dioxide ice particles, and they extend to the surface.

"By observing this way, the Mars Climate Sounder is able to distinguish the particles in the atmosphere from the dry ice on the surface," Kass said.

The Red Planet's south polar residual ice cap is the only place on Mars where frozen carbon dioxide exists on the surface year-round. It is still unclear how the carbon dioxide from Mars' atmosphere gets deposited.

"The finding of snowfall could mean that the type of deposition -- snow or frost -- is somehow linked to the year-to-year preservation of the residual cap," Hayne said.